cleaning up videos quailty

I recently downloaded a short video clip .Its very small in physical size-about the size of a postage stamp in the middle of the screen. When I increase the size it gets grainy and hard to make out. Does anybody know how to clean up this video or perhaps a program that will do the job. I'm running Windows XP Media Center Edtion. FYI-its a clean video clip!

impossible. it is easy to make something with excelleant quality horrible quality but basically impossible to do the opposite. would probably be cheaper to higher actors and actresses and record the small clip yourself :-P